1876.] 27 



III. — On certain protracted Irregularities of Atmospheric Pressure in the 

 Indian Monsoon-region, and their Relation to Variations of the Local 

 Rainfall. — By Hejstit F. Blaotoed, F. Q. S. 



(Received May 13th ;— Read June~7th, 1875.) 



JVote. — The greater part of the following paper was written in 1874 for 

 communication to the British Association meeting at Belfast, and an ab- 

 stract of the pajDer appears in the Reports of the work of Section A for that 

 year. I did not, however, publish the paper in extenso, as I desired before 

 doing so to verify the conclusions by some further experience. The origi- 

 nal paper discussed the phenomena of the years 1868, 1871, 1872, and 

 1873 : I have now added those of 1869, 1870, and 1874 ; and I have re- 

 drawn the tables of the former years, substituting as the standard of 

 comparison, the averages obtained up to the end of 1874, for those up to 

 the end of 1873 oidy. The result has been a further confirmation of at 

 least one of my conclusions, viz. the persistency of certain anomalies of 

 pressure distribution. The other conclusion suggested, viz. that the rainfall 

 of each season is influenced in a characteristic manner by these anomalous 

 variations of the pressure, is one that requires for its verification a far 

 more detailed and prolonged study than the data here given will admit of. 



In a paper read in February 1874 before the Royal Society, I con- 

 cluded from a detailed discussion of the wind-directions, and the distribution 

 of atmospheric pressure in Northern and Central India, as well as of other 

 meteorological elements, that the Indian branches of the two monsoons, in 

 the one case originate, in the other terminate south of the Himalaya ; and 

 that they are but little, if at all, dependent on the variations of atmospheric 

 pressure in Central Asia, to which they have generally been attributed. 

 The great mountain chain acts in fact as a complete barrier to the lower 

 half of the atmosphere, and it was shewn that it is within this stratum that 

 the alternating air-currents are restricted. From April to the end of Sep- 

 tember an area of low pressure exists over a part of Central India and the 

 Punjab, towards which a tolerably steady current blows from equatorial seas ; 

 while, during the remainder of the year, there is an area of less intense maxi- 

 mum pressure in approximately the same region, in whicb the winter or NE 

 monsoon originates. In the charts given in illustration of this paper, it was 

 shewn that the position of the barometric minimmn in the SW monsoon, 

 on an average, changes but little from the month of May, before the rains 

 set in, up to the end of the rains in September or October ; when the pres- 

 sure becomes nearly uniform, prior to the re-distribution which characterises 

 the winter season. The object of the present paper is to shew that while 



